I'm too late to the party on this one, but I'd suggest 125,000 gp for the selling price of a trained and amicable level 11 iron dragon.
I gain this by reverse engineering the price of mounts in the following fashion.
1) mounts generally sell for the same as an item of 2 levels higher than the mount's level
2) mounts with exceptional capacities however, sell for higher. In particular, flying mounts seem to sell for the same as an item of 4 levels higher than the mount's level.
3) While the game has no precedent for owning a mount that is a solo, it seems reasonable that it should be worth 5 times the price of a standard mount of it's level. (the one elite mount, the manticore, is worth approximately twice what a standard mount of it's level would be worth).
4) Level 11 Iron Dragon +4 = level 15 item (25,000 gp). 25,000 gp x 5 =125,000 gp.
Obviously, an untrained and obviously unwilling dragon should be worth substantially less (perhaps 1/5 or less the normal price). Other posters have already addressed these issues, however. I'd also add that I'd never allow a dragon to be purchased except under the most unusual of circumstances, and that the price above is calculated from a game balance perspective only.